A load-splitting transmission normally has a central shaft having relatively rotatable input and output ends, and a pair of outer or satellite shafts flanking this central shaft. Torque is transmitted from the input end of the central shaft through appropriate gearing either to both of the satellite shafts, or to one of the shafts and thence through further gearing to the other shaft. Gearing is provided for synchronous rotation of both of the outer shafts which are in turn connected through another set of gears to the output end of the transmission. Several sets of gears are provided in such a transmission which can be switched in and out by means of clutches to obtain different transmission ratios.
With such a transmission the input torque is therefore split between the satellite shafts so that wear is distributed over several parts. Furthermore such splitting of load allows the forces within the transmission frequently to be balanced against each other.
In order to compensate for manufacturing tolerances in the gears it has been found necessary to mount the central shaft so that it floats. Thus limited radial movement of this central shaft is possible when the transmission is shifted to allow the various gears to come into mesh with one another and to compensate for poor manufacturing tolerances.
As a result of this type of construction using a floating central shaft it is impossible to employ standard synchromesh clutches for the various shifting operations, as such clutches require exact alignment of the various gears and shafts they act on. For this reason cruder jaw-type clutches are used which allow limited radial displacement of the clutched parts relative to each other. The obvious result of this type of construction is that shifting with such a transmission must be done with extreme care to prevent clashing of the gears.
Another disadvantage of such a system is that it is necessary to provide at least one pair of gears for each transmission speed. Thus the size of such a transmission, as well as the cost, remains elevated.